The present invention relates to a method for the measurement of web tension.
The present invention also concerns an apparatus for the implementation of the method.
When a web, e.g., a paper web, is conveyed from one roll to another, the rotation speeds and consequently, the rotating torques of the roll drives must be controlled in a proper relation to each other, since the diameter of the rotated roll is linearly related to the tangential speed at the roll perimeter. In order to make it possible to wind the web onto the roll so that the web tension is maintained proper throughout the entire roll thickness and to be able to wind the web from one roll to another so that the web will not be slackened or tightened excessively with resultant breaks, the web tension must be measured during the wind/unwind operation and the rotation speeds of rolls adjusted according to the measured tension. The tension measurement apparatus also quickly indicates web breaks.
Conventional methods used for web tension measurement are the following:
Web tension measurement can be implemented using the method illustrated in FIG. 1a, in which compressed air is blown against the web at constant pressure and the web is conveyed over the gauging probe. The gauging probe is provided with a hole, in which the induced pressure is proportional to the web tension, whereby information on web tension can be obtained from this pressure. This method is inaccurate and requires the gauging probe to be located in such a place within the machine that allows the web to be conveyed at a substantially acute angle over the gauging probe, to allow a sufficient measurement counterpressure to be formed. This kind of an acute deflection in the web transport direction imposes a stress on the web and increases its tendency to break.
Further, web tension can be measured by pressing against the web in a transverse direction to its machine direction, for instance, using a freely riding roller as illustrated in FIG. 1b. Because in this manner a force component related to the web tension and the forced change in the web direction is imposed on the web, the web tension can be measured by placing a strain gauge at a suitable place, e.g. at the middle of the support beam span of the measurement roller, or alternatively, at the end of the roller. If the strain components of the roller support structure are defined with a high accuracy for different stress conditions, the result obtained is accurate. In other aspects, this system inherently bears the same drawbacks as the first method described.
The FI patent publication 62419 discloses a measurement method based on the propagation velocity of a planar wave as illustrated in FIG. 1c. According to this method, a disturbance is excited on the web, which in this case is an acoustic wave generated with the help of a loudspeaker; and the wave propagation time is measured with a microphone. The propagation velocity of the disturbance is related to the tension of the web, which provides for the determination of web tension when the other factors such as the base weight, temperature and Young's modulus stay constant. Because of the acoustic method used, interference from industrial building noise and other exiting disturbance will cause error in the measurement result.
U.S. Pat. Nos. 2,755,032 and 3,057,574 describe mechanical and compressed-air operated apparatus for web tension measurement and control. According to these methods, the web is pressed by two pads, whereby the web is deflected and a force proportional to the web tension is imposed on the pads. The mechanical measurement systems of web tension impose a heavy load on the web and, consequently, are not suitable for modern web processing machines of high speed. An embodiment of such apparatus is illustrated in FIG. 1d.